NHS chief Sir David Nicholson has angrily denied the suggestion he has been involved in a cover-up over payments silencing health workers.

The NHS chief executive insisted he had not misled MPs earlier this year by saying he was only aware of one gagging order.

In fresh evidence to the Public Accounts Committee, he said he had backed whistle-blowers and changed the system to help them.

His comments came after new figures showed hospitals spent £2m on more than 50 gagging orders stopping staff from speaking out.

Sir David told the committee in March that he was only aware of one such order, worth £500,000.

Tory MP Steve Barclay, who obtained the data, claimed Sir David had either failed to ask questions or was "complicit in a cover-up".

He called for the executive, who is already due to retire next year, to stand down immediately to allow for a culture change.

But Sir David told the Commons' spending watchdog: "I can absolutely refute that I have ever been involved in any kind of cover-up in relation to the expenditure that has been identified.

"I have always supported people who have spoken out against the system. It is an important part of being a health professional and being a leader in the NHS.

"To connect me with a cover-up, I think is entirely and utterly inappropriate and I completely refute it."

Image:Sir David Nicholson giving evidence on Wednesday

At least 52 staff have been silenced since 2008, according to details released following Mr Barclay's Freedom of Information Act request.

The orders - some of which cost as much as £500,000 - are all thought to contain confidentiality clauses.

Speaking after the hearing, Sir David refused to tell reporters how many gagging orders had been used but said it had been made "absolutely clear" that such orders are "entirely inappropriate".

Earlier, Mr Barclay told Sky News: "He gave an assurance to Parliament in March that he would investigate this and we discover he has done absolutely nothing.

"Given his own deputy knew about this, given around a fifth of hospitals have made these payments, it is very hard to believe that he didn't know - and then the question is why didn't he - or he did know in which case he has not been straight with Parliament."

He added: "We need to change the culture in the NHS and we need a culture where people feel confident about speaking out about problems.

"There is movement within the NHS but I think Sir David is an impediment to that and the sooner he goes the better."

The £2m in payouts is on top of the £15m it emerged in March had been spent by NHS hospital trusts on silencing almost 600 staff.

That figure did not include "judicially mediated" settlements, under which hospitals agree a deal with staff and then have it signed off by a judge or senior lawyer.

Because they are not passed to the Department of Health or Treasury, the Government had no chance to block them.

Health Secretary Jeremy Hunt banned the use of gagging clauses in compromise settlements after the revelations.

Following the new claims, the Department of Health said the system had been changed so that all severance payments were properly scrutinised and staff were made aware of their whistle-blowing rights.

"Judicial mediation payments do not mean that someone is gagged - it is a way of resolving a dispute and suitable cases for this are decided on by a judge," a spokesman said.